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  <div class="section" id="curses-programming-with-python">
<span id="curses-howto"></span><h1>Curses Programming with Python<a class="headerlink" href="#curses-programming-with-python" title="Permalink to this headline">¶</a></h1>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name">Author:</th><td class="field-body">A.M. Kuchling, Eric S. Raymond</td>
</tr>
<tr class="field-even field"><th class="field-name">Release:</th><td class="field-body">2.04</td>
</tr>
</tbody>
</table>
<div class="topic">
<p class="topic-title first">Abstract</p>
<p>This document describes how to use the <a class="reference internal" href="../library/curses.html#module-curses" title="curses: An interface to the curses library, providing portable terminal handling. (Unix)"><code class="xref py py-mod docutils literal"><span class="pre">curses</span></code></a> extension
module to control text-mode displays.</p>
</div>
<div class="section" id="what-is-curses">
<h2>What is curses?<a class="headerlink" href="#what-is-curses" title="Permalink to this headline">¶</a></h2>
<p>The curses library supplies a terminal-independent screen-painting and
keyboard-handling facility for text-based terminals; such terminals
include VT100s, the Linux console, and the simulated terminal provided
by various programs.  Display terminals support various control codes
to perform common operations such as moving the cursor, scrolling the
screen, and erasing areas.  Different terminals use widely differing
codes, and often have their own minor quirks.</p>
<p>In a world of graphical displays, one might ask “why bother”?  It’s
true that character-cell display terminals are an obsolete technology,
but there are niches in which being able to do fancy things with them
are still valuable.  One niche is on small-footprint or embedded
Unixes that don’t run an X server.  Another is tools such as OS
installers and kernel configurators that may have to run before any
graphical support is available.</p>
<p>The curses library provides fairly basic functionality, providing the
programmer with an abstraction of a display containing multiple
non-overlapping windows of text.  The contents of a window can be
changed in various ways—adding text, erasing it, changing its
appearance—and the curses library will figure out what control codes
need to be sent to the terminal to produce the right output.  curses
doesn’t provide many user-interface concepts such as buttons, checkboxes,
or dialogs; if you need such features, consider a user interface library such as
<a class="reference external" href="https://pypi.python.org/pypi/urwid/">Urwid</a>.</p>
<p>The curses library was originally written for BSD Unix; the later System V
versions of Unix from AT&amp;T added many enhancements and new functions. BSD curses
is no longer maintained, having been replaced by ncurses, which is an
open-source implementation of the AT&amp;T interface.  If you’re using an
open-source Unix such as Linux or FreeBSD, your system almost certainly uses
ncurses.  Since most current commercial Unix versions are based on System V
code, all the functions described here will probably be available.  The older
versions of curses carried by some proprietary Unixes may not support
everything, though.</p>
<p>The Windows version of Python doesn’t include the <a class="reference internal" href="../library/curses.html#module-curses" title="curses: An interface to the curses library, providing portable terminal handling. (Unix)"><code class="xref py py-mod docutils literal"><span class="pre">curses</span></code></a>
module.  A ported version called <a class="reference external" href="https://pypi.python.org/pypi/UniCurses">UniCurses</a> is available.  You could
also try <a class="reference external" href="http://effbot.org/zone/console-index.htm">the Console module</a>
written by Fredrik Lundh, which doesn’t
use the same API as curses but provides cursor-addressable text output
and full support for mouse and keyboard input.</p>
<div class="section" id="the-python-curses-module">
<h3>The Python curses module<a class="headerlink" href="#the-python-curses-module" title="Permalink to this headline">¶</a></h3>
<p>The Python module is a fairly simple wrapper over the C functions provided by
curses; if you’re already familiar with curses programming in C, it’s really
easy to transfer that knowledge to Python.  The biggest difference is that the
Python interface makes things simpler by merging different C functions such as
<code class="xref c c-func docutils literal"><span class="pre">addstr()</span></code>, <code class="xref c c-func docutils literal"><span class="pre">mvaddstr()</span></code>, and <code class="xref c c-func docutils literal"><span class="pre">mvwaddstr()</span></code> into a single
<a class="reference internal" href="../library/curses.html#curses.window.addstr" title="curses.window.addstr"><code class="xref py py-meth docutils literal"><span class="pre">addstr()</span></code></a> method.  You’ll see this covered in more
detail later.</p>
<p>This HOWTO is an introduction to writing text-mode programs with curses
and Python. It doesn’t attempt to be a complete guide to the curses API; for
that, see the Python library guide’s section on ncurses, and the C manual pages
for ncurses.  It will, however, give you the basic ideas.</p>
</div>
</div>
<div class="section" id="starting-and-ending-a-curses-application">
<h2>Starting and ending a curses application<a class="headerlink" href="#starting-and-ending-a-curses-application" title="Permalink to this headline">¶</a></h2>
<p>Before doing anything, curses must be initialized.  This is done by
calling the <a class="reference internal" href="../library/curses.html#curses.initscr" title="curses.initscr"><code class="xref py py-func docutils literal"><span class="pre">initscr()</span></code></a> function, which will determine the
terminal type, send any required setup codes to the terminal, and
create various internal data structures.  If successful,
<code class="xref py py-func docutils literal"><span class="pre">initscr()</span></code> returns a window object representing the entire
screen; this is usually called <code class="docutils literal"><span class="pre">stdscr</span></code> after the name of the
corresponding C variable.</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">curses</span>
<span class="n">stdscr</span> <span class="o">=</span> <span class="n">curses</span><span class="o">.</span><span class="n">initscr</span><span class="p">()</span>
</pre></div>
</div>
<p>Usually curses applications turn off automatic echoing of keys to the
screen, in order to be able to read keys and only display them under
certain circumstances.  This requires calling the
<a class="reference internal" href="../library/curses.html#curses.noecho" title="curses.noecho"><code class="xref py py-func docutils literal"><span class="pre">noecho()</span></code></a> function.</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">curses</span><span class="o">.</span><span class="n">noecho</span><span class="p">()</span>
</pre></div>
</div>
<p>Applications will also commonly need to react to keys instantly,
without requiring the Enter key to be pressed; this is called cbreak
mode, as opposed to the usual buffered input mode.</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">curses</span><span class="o">.</span><span class="n">cbreak</span><span class="p">()</span>
</pre></div>
</div>
<p>Terminals usually return special keys, such as the cursor keys or navigation
keys such as Page Up and Home, as a multibyte escape sequence.  While you could
write your application to expect such sequences and process them accordingly,
curses can do it for you, returning a special value such as
<code class="xref py py-const docutils literal"><span class="pre">curses.KEY_LEFT</span></code>.  To get curses to do the job, you’ll have to enable
keypad mode.</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">stdscr</span><span class="o">.</span><span class="n">keypad</span><span class="p">(</span><span class="kc">True</span><span class="p">)</span>
</pre></div>
</div>
<p>Terminating a curses application is much easier than starting one. You’ll need
to call:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">curses</span><span class="o">.</span><span class="n">nocbreak</span><span class="p">()</span>
<span class="n">stdscr</span><span class="o">.</span><span class="n">keypad</span><span class="p">(</span><span class="kc">False</span><span class="p">)</span>
<span class="n">curses</span><span class="o">.</span><span class="n">echo</span><span class="p">()</span>
</pre></div>
</div>
<p>to reverse the curses-friendly terminal settings. Then call the
<a class="reference internal" href="../library/curses.html#curses.endwin" title="curses.endwin"><code class="xref py py-func docutils literal"><span class="pre">endwin()</span></code></a> function to restore the terminal to its original
operating mode.</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">curses</span><span class="o">.</span><span class="n">endwin</span><span class="p">()</span>
</pre></div>
</div>
<p>A common problem when debugging a curses application is to get your terminal
messed up when the application dies without restoring the terminal to its
previous state.  In Python this commonly happens when your code is buggy and
raises an uncaught exception.  Keys are no longer echoed to the screen when
you type them, for example, which makes using the shell difficult.</p>
<p>In Python you can avoid these complications and make debugging much easier by
importing the <a class="reference internal" href="../library/curses.html#curses.wrapper" title="curses.wrapper"><code class="xref py py-func docutils literal"><span class="pre">curses.wrapper()</span></code></a> function and using it like this:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">curses</span> <span class="k">import</span> <span class="n">wrapper</span>

<span class="k">def</span> <span class="nf">main</span><span class="p">(</span><span class="n">stdscr</span><span class="p">):</span>
    <span class="c1"># Clear screen</span>
    <span class="n">stdscr</span><span class="o">.</span><span class="n">clear</span><span class="p">()</span>

    <span class="c1"># This raises ZeroDivisionError when i == 10.</span>
    <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">11</span><span class="p">):</span>
        <span class="n">v</span> <span class="o">=</span> <span class="n">i</span><span class="o">-</span><span class="mi">10</span>
        <span class="n">stdscr</span><span class="o">.</span><span class="n">addstr</span><span class="p">(</span><span class="n">i</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="s1">&#39;10 divided by </span><span class="si">{}</span><span class="s1"> is </span><span class="si">{}</span><span class="s1">&#39;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">v</span><span class="p">,</span> <span class="mi">10</span><span class="o">/</span><span class="n">v</span><span class="p">))</span>

    <span class="n">stdscr</span><span class="o">.</span><span class="n">refresh</span><span class="p">()</span>
    <span class="n">stdscr</span><span class="o">.</span><span class="n">getkey</span><span class="p">()</span>

<span class="n">wrapper</span><span class="p">(</span><span class="n">main</span><span class="p">)</span>
</pre></div>
</div>
<p>The <a class="reference internal" href="../library/curses.html#curses.wrapper" title="curses.wrapper"><code class="xref py py-func docutils literal"><span class="pre">wrapper()</span></code></a> function takes a callable object and does the
initializations described above, also initializing colors if color
support is present.  <code class="xref py py-func docutils literal"><span class="pre">wrapper()</span></code> then runs your provided callable.
Once the callable returns, <code class="xref py py-func docutils literal"><span class="pre">wrapper()</span></code> will restore the original
state of the terminal.  The callable is called inside a
<a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal"><span class="pre">try</span></code></a>…<a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal"><span class="pre">except</span></code></a> that catches exceptions, restores
the state of the terminal, and then re-raises the exception.  Therefore
your terminal won’t be left in a funny state on exception and you’ll be
able to read the exception’s message and traceback.</p>
</div>
<div class="section" id="windows-and-pads">
<h2>Windows and Pads<a class="headerlink" href="#windows-and-pads" title="Permalink to this headline">¶</a></h2>
<p>Windows are the basic abstraction in curses.  A window object represents a
rectangular area of the screen, and supports methods to display text,
erase it, allow the user to input strings, and so forth.</p>
<p>The <code class="docutils literal"><span class="pre">stdscr</span></code> object returned by the <a class="reference internal" href="../library/curses.html#curses.initscr" title="curses.initscr"><code class="xref py py-func docutils literal"><span class="pre">initscr()</span></code></a> function is a
window object that covers the entire screen.  Many programs may need
only this single window, but you might wish to divide the screen into
smaller windows, in order to redraw or clear them separately. The
<a class="reference internal" href="../library/curses.html#curses.newwin" title="curses.newwin"><code class="xref py py-func docutils literal"><span class="pre">newwin()</span></code></a> function creates a new window of a given size,
returning the new window object.</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">begin_x</span> <span class="o">=</span> <span class="mi">20</span><span class="p">;</span> <span class="n">begin_y</span> <span class="o">=</span> <span class="mi">7</span>
<span class="n">height</span> <span class="o">=</span> <span class="mi">5</span><span class="p">;</span> <span class="n">width</span> <span class="o">=</span> <span class="mi">40</span>
<span class="n">win</span> <span class="o">=</span> <span class="n">curses</span><span class="o">.</span><span class="n">newwin</span><span class="p">(</span><span class="n">height</span><span class="p">,</span> <span class="n">width</span><span class="p">,</span> <span class="n">begin_y</span><span class="p">,</span> <span class="n">begin_x</span><span class="p">)</span>
</pre></div>
</div>
<p>Note that the coordinate system used in curses is unusual.
Coordinates are always passed in the order <em>y,x</em>, and the top-left
corner of a window is coordinate (0,0).  This breaks the normal
convention for handling coordinates where the <em>x</em> coordinate comes
first.  This is an unfortunate difference from most other computer
applications, but it’s been part of curses since it was first written,
and it’s too late to change things now.</p>
<p>Your application can determine the size of the screen by using the
<code class="xref py py-data docutils literal"><span class="pre">curses.LINES</span></code> and <code class="xref py py-data docutils literal"><span class="pre">curses.COLS</span></code> variables to obtain the <em>y</em> and
<em>x</em> sizes.  Legal coordinates will then extend from <code class="docutils literal"><span class="pre">(0,0)</span></code> to
<code class="docutils literal"><span class="pre">(curses.LINES</span> <span class="pre">-</span> <span class="pre">1,</span> <span class="pre">curses.COLS</span> <span class="pre">-</span> <span class="pre">1)</span></code>.</p>
<p>When you call a method to display or erase text, the effect doesn’t
immediately show up on the display.  Instead you must call the
<a class="reference internal" href="../library/curses.html#curses.window.refresh" title="curses.window.refresh"><code class="xref py py-meth docutils literal"><span class="pre">refresh()</span></code></a> method of window objects to update the
screen.</p>
<p>This is because curses was originally written with slow 300-baud
terminal connections in mind; with these terminals, minimizing the
time required to redraw the screen was very important.  Instead curses
accumulates changes to the screen and displays them in the most
efficient manner when you call <code class="xref py py-meth docutils literal"><span class="pre">refresh()</span></code>.  For example, if your
program displays some text in a window and then clears the window,
there’s no need to send the original text because they’re never
visible.</p>
<p>In practice, explicitly telling curses to redraw a window doesn’t
really complicate programming with curses much. Most programs go into a flurry
of activity, and then pause waiting for a keypress or some other action on the
part of the user.  All you have to do is to be sure that the screen has been
redrawn before pausing to wait for user input, by first calling
<code class="docutils literal"><span class="pre">stdscr.refresh()</span></code> or the <code class="xref py py-meth docutils literal"><span class="pre">refresh()</span></code> method of some other relevant
window.</p>
<p>A pad is a special case of a window; it can be larger than the actual display
screen, and only a portion of the pad displayed at a time. Creating a pad
requires the pad’s height and width, while refreshing a pad requires giving the
coordinates of the on-screen area where a subsection of the pad will be
displayed.</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">pad</span> <span class="o">=</span> <span class="n">curses</span><span class="o">.</span><span class="n">newpad</span><span class="p">(</span><span class="mi">100</span><span class="p">,</span> <span class="mi">100</span><span class="p">)</span>
<span class="c1"># These loops fill the pad with letters; addch() is</span>
<span class="c1"># explained in the next section</span>
<span class="k">for</span> <span class="n">y</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">99</span><span class="p">):</span>
    <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">99</span><span class="p">):</span>
        <span class="n">pad</span><span class="o">.</span><span class="n">addch</span><span class="p">(</span><span class="n">y</span><span class="p">,</span><span class="n">x</span><span class="p">,</span> <span class="nb">ord</span><span class="p">(</span><span class="s1">&#39;a&#39;</span><span class="p">)</span> <span class="o">+</span> <span class="p">(</span><span class="n">x</span><span class="o">*</span><span class="n">x</span><span class="o">+</span><span class="n">y</span><span class="o">*</span><span class="n">y</span><span class="p">)</span> <span class="o">%</span> <span class="mi">26</span><span class="p">)</span>

<span class="c1"># Displays a section of the pad in the middle of the screen.</span>
<span class="c1"># (0,0) : coordinate of upper-left corner of pad area to display.</span>
<span class="c1"># (5,5) : coordinate of upper-left corner of window area to be filled</span>
<span class="c1">#         with pad content.</span>
<span class="c1"># (20, 75) : coordinate of lower-right corner of window area to be</span>
<span class="c1">#          : filled with pad content.</span>
<span class="n">pad</span><span class="o">.</span><span class="n">refresh</span><span class="p">(</span> <span class="mi">0</span><span class="p">,</span><span class="mi">0</span><span class="p">,</span> <span class="mi">5</span><span class="p">,</span><span class="mi">5</span><span class="p">,</span> <span class="mi">20</span><span class="p">,</span><span class="mi">75</span><span class="p">)</span>
</pre></div>
</div>
<p>The <code class="xref py py-meth docutils literal"><span class="pre">refresh()</span></code> call displays a section of the pad in the rectangle
extending from coordinate (5,5) to coordinate (20,75) on the screen; the upper
left corner of the displayed section is coordinate (0,0) on the pad.  Beyond
that difference, pads are exactly like ordinary windows and support the same
methods.</p>
<p>If you have multiple windows and pads on screen there is a more
efficient way to update the screen and prevent annoying screen flicker
as each part of the screen gets updated.  <code class="xref py py-meth docutils literal"><span class="pre">refresh()</span></code> actually
does two things:</p>
<ol class="arabic simple">
<li>Calls the <a class="reference internal" href="../library/curses.html#curses.window.noutrefresh" title="curses.window.noutrefresh"><code class="xref py py-meth docutils literal"><span class="pre">noutrefresh()</span></code></a> method of each window
to update an underlying data structure representing the desired
state of the screen.</li>
<li>Calls the function <a class="reference internal" href="../library/curses.html#curses.doupdate" title="curses.doupdate"><code class="xref py py-func docutils literal"><span class="pre">doupdate()</span></code></a> function to change the
physical screen to match the desired state recorded in the data structure.</li>
</ol>
<p>Instead you can call <code class="xref py py-meth docutils literal"><span class="pre">noutrefresh()</span></code> on a number of windows to
update the data structure, and then call <code class="xref py py-func docutils literal"><span class="pre">doupdate()</span></code> to update
the screen.</p>
</div>
<div class="section" id="displaying-text">
<h2>Displaying Text<a class="headerlink" href="#displaying-text" title="Permalink to this headline">¶</a></h2>
<p>From a C programmer’s point of view, curses may sometimes look like a
twisty maze of functions, all subtly different.  For example,
<code class="xref c c-func docutils literal"><span class="pre">addstr()</span></code> displays a string at the current cursor location in
the <code class="docutils literal"><span class="pre">stdscr</span></code> window, while <code class="xref c c-func docutils literal"><span class="pre">mvaddstr()</span></code> moves to a given y,x
coordinate first before displaying the string. <code class="xref c c-func docutils literal"><span class="pre">waddstr()</span></code> is just
like <code class="xref c c-func docutils literal"><span class="pre">addstr()</span></code>, but allows specifying a window to use instead of
using <code class="docutils literal"><span class="pre">stdscr</span></code> by default. <code class="xref c c-func docutils literal"><span class="pre">mvwaddstr()</span></code> allows specifying both
a window and a coordinate.</p>
<p>Fortunately the Python interface hides all these details.  <code class="docutils literal"><span class="pre">stdscr</span></code>
is a window object like any other, and methods such as
<a class="reference internal" href="../library/curses.html#curses.window.addstr" title="curses.window.addstr"><code class="xref py py-meth docutils literal"><span class="pre">addstr()</span></code></a> accept multiple argument forms.  Usually there
are four different forms.</p>
<table border="1" class="docutils">
<colgroup>
<col width="41%" />
<col width="59%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Form</th>
<th class="head">Description</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><em>str</em> or <em>ch</em></td>
<td>Display the string <em>str</em> or character <em>ch</em> at
the current position</td>
</tr>
<tr class="row-odd"><td><em>str</em> or <em>ch</em>, <em>attr</em></td>
<td>Display the string <em>str</em> or character <em>ch</em>,
using attribute <em>attr</em> at the current
position</td>
</tr>
<tr class="row-even"><td><em>y</em>, <em>x</em>, <em>str</em> or <em>ch</em></td>
<td>Move to position <em>y,x</em> within the window, and
display <em>str</em> or <em>ch</em></td>
</tr>
<tr class="row-odd"><td><em>y</em>, <em>x</em>, <em>str</em> or <em>ch</em>, <em>attr</em></td>
<td>Move to position <em>y,x</em> within the window, and
display <em>str</em> or <em>ch</em>, using attribute <em>attr</em></td>
</tr>
</tbody>
</table>
<p>Attributes allow displaying text in highlighted forms such as boldface,
underline, reverse code, or in color.  They’ll be explained in more detail in
the next subsection.</p>
<p>The <a class="reference internal" href="../library/curses.html#curses.window.addstr" title="curses.window.addstr"><code class="xref py py-meth docutils literal"><span class="pre">addstr()</span></code></a> method takes a Python string or
bytestring as the value to be displayed.  The contents of bytestrings
are sent to the terminal as-is.  Strings are encoded to bytes using
the value of the window’s <code class="xref py py-attr docutils literal"><span class="pre">encoding</span></code> attribute; this defaults to
the default system encoding as returned by
<a class="reference internal" href="../library/locale.html#locale.getpreferredencoding" title="locale.getpreferredencoding"><code class="xref py py-func docutils literal"><span class="pre">locale.getpreferredencoding()</span></code></a>.</p>
<p>The <a class="reference internal" href="../library/curses.html#curses.window.addch" title="curses.window.addch"><code class="xref py py-meth docutils literal"><span class="pre">addch()</span></code></a> methods take a character, which can be
either a string of length 1, a bytestring of length 1, or an integer.</p>
<p>Constants are provided for extension characters; these constants are
integers greater than 255.  For example, <code class="xref py py-const docutils literal"><span class="pre">ACS_PLMINUS</span></code> is a +/-
symbol, and <code class="xref py py-const docutils literal"><span class="pre">ACS_ULCORNER</span></code> is the upper left corner of a box
(handy for drawing borders).  You can also use the appropriate Unicode
character.</p>
<p>Windows remember where the cursor was left after the last operation, so if you
leave out the <em>y,x</em> coordinates, the string or character will be displayed
wherever the last operation left off.  You can also move the cursor with the
<code class="docutils literal"><span class="pre">move(y,x)</span></code> method.  Because some terminals always display a flashing cursor,
you may want to ensure that the cursor is positioned in some location where it
won’t be distracting; it can be confusing to have the cursor blinking at some
apparently random location.</p>
<p>If your application doesn’t need a blinking cursor at all, you can
call <code class="docutils literal"><span class="pre">curs_set(False)</span></code> to make it invisible.  For compatibility
with older curses versions, there’s a <code class="docutils literal"><span class="pre">leaveok(bool)</span></code> function
that’s a synonym for <a class="reference internal" href="../library/curses.html#curses.curs_set" title="curses.curs_set"><code class="xref py py-func docutils literal"><span class="pre">curs_set()</span></code></a>.  When <em>bool</em> is true, the
curses library will attempt to suppress the flashing cursor, and you
won’t need to worry about leaving it in odd locations.</p>
<div class="section" id="attributes-and-color">
<h3>Attributes and Color<a class="headerlink" href="#attributes-and-color" title="Permalink to this headline">¶</a></h3>
<p>Characters can be displayed in different ways.  Status lines in a text-based
application are commonly shown in reverse video, or a text viewer may need to
highlight certain words.  curses supports this by allowing you to specify an
attribute for each cell on the screen.</p>
<p>An attribute is an integer, each bit representing a different
attribute.  You can try to display text with multiple attribute bits
set, but curses doesn’t guarantee that all the possible combinations
are available, or that they’re all visually distinct.  That depends on
the ability of the terminal being used, so it’s safest to stick to the
most commonly available attributes, listed here.</p>
<table border="1" class="docutils">
<colgroup>
<col width="37%" />
<col width="63%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">Attribute</th>
<th class="head">Description</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td><code class="xref py py-const docutils literal"><span class="pre">A_BLINK</span></code></td>
<td>Blinking text</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-const docutils literal"><span class="pre">A_BOLD</span></code></td>
<td>Extra bright or bold text</td>
</tr>
<tr class="row-even"><td><code class="xref py py-const docutils literal"><span class="pre">A_DIM</span></code></td>
<td>Half bright text</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-const docutils literal"><span class="pre">A_REVERSE</span></code></td>
<td>Reverse-video text</td>
</tr>
<tr class="row-even"><td><code class="xref py py-const docutils literal"><span class="pre">A_STANDOUT</span></code></td>
<td>The best highlighting mode available</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-const docutils literal"><span class="pre">A_UNDERLINE</span></code></td>
<td>Underlined text</td>
</tr>
</tbody>
</table>
<p>So, to display a reverse-video status line on the top line of the screen, you
could code:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">stdscr</span><span class="o">.</span><span class="n">addstr</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="s2">&quot;Current mode: Typing mode&quot;</span><span class="p">,</span>
              <span class="n">curses</span><span class="o">.</span><span class="n">A_REVERSE</span><span class="p">)</span>
<span class="n">stdscr</span><span class="o">.</span><span class="n">refresh</span><span class="p">()</span>
</pre></div>
</div>
<p>The curses library also supports color on those terminals that provide it. The
most common such terminal is probably the Linux console, followed by color
xterms.</p>
<p>To use color, you must call the <a class="reference internal" href="../library/curses.html#curses.start_color" title="curses.start_color"><code class="xref py py-func docutils literal"><span class="pre">start_color()</span></code></a> function soon
after calling <a class="reference internal" href="../library/curses.html#curses.initscr" title="curses.initscr"><code class="xref py py-func docutils literal"><span class="pre">initscr()</span></code></a>, to initialize the default color set
(the <a class="reference internal" href="../library/curses.html#curses.wrapper" title="curses.wrapper"><code class="xref py py-func docutils literal"><span class="pre">curses.wrapper()</span></code></a> function does this automatically).  Once that’s
done, the <a class="reference internal" href="../library/curses.html#curses.has_colors" title="curses.has_colors"><code class="xref py py-func docutils literal"><span class="pre">has_colors()</span></code></a> function returns TRUE if the terminal
in use can
actually display color.  (Note: curses uses the American spelling ‘color’,
instead of the Canadian/British spelling ‘colour’.  If you’re used to the
British spelling, you’ll have to resign yourself to misspelling it for the sake
of these functions.)</p>
<p>The curses library maintains a finite number of color pairs, containing a
foreground (or text) color and a background color.  You can get the attribute
value corresponding to a color pair with the <a class="reference internal" href="../library/curses.html#curses.color_pair" title="curses.color_pair"><code class="xref py py-func docutils literal"><span class="pre">color_pair()</span></code></a>
function; this can be bitwise-OR’ed with other attributes such as
<code class="xref py py-const docutils literal"><span class="pre">A_REVERSE</span></code>, but again, such combinations are not guaranteed to work
on all terminals.</p>
<p>An example, which displays a line of text using color pair 1:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">stdscr</span><span class="o">.</span><span class="n">addstr</span><span class="p">(</span><span class="s2">&quot;Pretty text&quot;</span><span class="p">,</span> <span class="n">curses</span><span class="o">.</span><span class="n">color_pair</span><span class="p">(</span><span class="mi">1</span><span class="p">))</span>
<span class="n">stdscr</span><span class="o">.</span><span class="n">refresh</span><span class="p">()</span>
</pre></div>
</div>
<p>As I said before, a color pair consists of a foreground and background color.
The <code class="docutils literal"><span class="pre">init_pair(n,</span> <span class="pre">f,</span> <span class="pre">b)</span></code> function changes the definition of color pair <em>n</em>, to
foreground color f and background color b.  Color pair 0 is hard-wired to white
on black, and cannot be changed.</p>
<p>Colors are numbered, and <code class="xref py py-func docutils literal"><span class="pre">start_color()</span></code> initializes 8 basic
colors when it activates color mode.  They are: 0:black, 1:red,
2:green, 3:yellow, 4:blue, 5:magenta, 6:cyan, and 7:white.  The <a class="reference internal" href="../library/curses.html#module-curses" title="curses: An interface to the curses library, providing portable terminal handling. (Unix)"><code class="xref py py-mod docutils literal"><span class="pre">curses</span></code></a>
module defines named constants for each of these colors:
<code class="xref py py-const docutils literal"><span class="pre">curses.COLOR_BLACK</span></code>, <code class="xref py py-const docutils literal"><span class="pre">curses.COLOR_RED</span></code>, and so forth.</p>
<p>Let’s put all this together. To change color 1 to red text on a white
background, you would call:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">curses</span><span class="o">.</span><span class="n">init_pair</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="n">curses</span><span class="o">.</span><span class="n">COLOR_RED</span><span class="p">,</span> <span class="n">curses</span><span class="o">.</span><span class="n">COLOR_WHITE</span><span class="p">)</span>
</pre></div>
</div>
<p>When you change a color pair, any text already displayed using that color pair
will change to the new colors.  You can also display new text in this color
with:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">stdscr</span><span class="o">.</span><span class="n">addstr</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span><span class="mi">0</span><span class="p">,</span> <span class="s2">&quot;RED ALERT!&quot;</span><span class="p">,</span> <span class="n">curses</span><span class="o">.</span><span class="n">color_pair</span><span class="p">(</span><span class="mi">1</span><span class="p">))</span>
</pre></div>
</div>
<p>Very fancy terminals can change the definitions of the actual colors to a given
RGB value.  This lets you change color 1, which is usually red, to purple or
blue or any other color you like.  Unfortunately, the Linux console doesn’t
support this, so I’m unable to try it out, and can’t provide any examples.  You
can check if your terminal can do this by calling
<a class="reference internal" href="../library/curses.html#curses.can_change_color" title="curses.can_change_color"><code class="xref py py-func docutils literal"><span class="pre">can_change_color()</span></code></a>, which returns <code class="docutils literal"><span class="pre">True</span></code> if the capability is
there.  If you’re lucky enough to have such a talented terminal, consult your
system’s man pages for more information.</p>
</div>
</div>
<div class="section" id="user-input">
<h2>User Input<a class="headerlink" href="#user-input" title="Permalink to this headline">¶</a></h2>
<p>The C curses library offers only very simple input mechanisms. Python’s
<a class="reference internal" href="../library/curses.html#module-curses" title="curses: An interface to the curses library, providing portable terminal handling. (Unix)"><code class="xref py py-mod docutils literal"><span class="pre">curses</span></code></a> module adds a basic text-input widget.  (Other libraries
such as <a class="reference external" href="https://pypi.python.org/pypi/urwid/">Urwid</a> have more extensive
collections of widgets.)</p>
<p>There are two methods for getting input from a window:</p>
<ul class="simple">
<li><a class="reference internal" href="../library/curses.html#curses.window.getch" title="curses.window.getch"><code class="xref py py-meth docutils literal"><span class="pre">getch()</span></code></a> refreshes the screen and then waits for
the user to hit a key, displaying the key if <a class="reference internal" href="../library/curses.html#curses.echo" title="curses.echo"><code class="xref py py-func docutils literal"><span class="pre">echo()</span></code></a> has been
called earlier.  You can optionally specify a coordinate to which
the cursor should be moved before pausing.</li>
<li><a class="reference internal" href="../library/curses.html#curses.window.getkey" title="curses.window.getkey"><code class="xref py py-meth docutils literal"><span class="pre">getkey()</span></code></a> does the same thing but converts the
integer to a string.  Individual characters are returned as
1-character strings, and special keys such as function keys return
longer strings containing a key name such as <code class="docutils literal"><span class="pre">KEY_UP</span></code> or <code class="docutils literal"><span class="pre">^G</span></code>.</li>
</ul>
<p>It’s possible to not wait for the user using the
<a class="reference internal" href="../library/curses.html#curses.window.nodelay" title="curses.window.nodelay"><code class="xref py py-meth docutils literal"><span class="pre">nodelay()</span></code></a> window method. After <code class="docutils literal"><span class="pre">nodelay(True)</span></code>,
<code class="xref py py-meth docutils literal"><span class="pre">getch()</span></code> and <code class="xref py py-meth docutils literal"><span class="pre">getkey()</span></code> for the window become
non-blocking. To signal that no input is ready, <code class="xref py py-meth docutils literal"><span class="pre">getch()</span></code> returns
<code class="docutils literal"><span class="pre">curses.ERR</span></code> (a value of -1) and <code class="xref py py-meth docutils literal"><span class="pre">getkey()</span></code> raises an exception.
There’s also a <a class="reference internal" href="../library/curses.html#curses.halfdelay" title="curses.halfdelay"><code class="xref py py-func docutils literal"><span class="pre">halfdelay()</span></code></a> function, which can be used to (in
effect) set a timer on each <code class="xref py py-meth docutils literal"><span class="pre">getch()</span></code>; if no input becomes
available within a specified delay (measured in tenths of a second),
curses raises an exception.</p>
<p>The <code class="xref py py-meth docutils literal"><span class="pre">getch()</span></code> method returns an integer; if it’s between 0 and 255, it
represents the ASCII code of the key pressed.  Values greater than 255 are
special keys such as Page Up, Home, or the cursor keys. You can compare the
value returned to constants such as <code class="xref py py-const docutils literal"><span class="pre">curses.KEY_PPAGE</span></code>,
<code class="xref py py-const docutils literal"><span class="pre">curses.KEY_HOME</span></code>, or <code class="xref py py-const docutils literal"><span class="pre">curses.KEY_LEFT</span></code>.  The main loop of
your program may look something like this:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">while</span> <span class="kc">True</span><span class="p">:</span>
    <span class="n">c</span> <span class="o">=</span> <span class="n">stdscr</span><span class="o">.</span><span class="n">getch</span><span class="p">()</span>
    <span class="k">if</span> <span class="n">c</span> <span class="o">==</span> <span class="nb">ord</span><span class="p">(</span><span class="s1">&#39;p&#39;</span><span class="p">):</span>
        <span class="n">PrintDocument</span><span class="p">()</span>
    <span class="k">elif</span> <span class="n">c</span> <span class="o">==</span> <span class="nb">ord</span><span class="p">(</span><span class="s1">&#39;q&#39;</span><span class="p">):</span>
        <span class="k">break</span>  <span class="c1"># Exit the while loop</span>
    <span class="k">elif</span> <span class="n">c</span> <span class="o">==</span> <span class="n">curses</span><span class="o">.</span><span class="n">KEY_HOME</span><span class="p">:</span>
        <span class="n">x</span> <span class="o">=</span> <span class="n">y</span> <span class="o">=</span> <span class="mi">0</span>
</pre></div>
</div>
<p>The <a class="reference internal" href="../library/curses.ascii.html#module-curses.ascii" title="curses.ascii: Constants and set-membership functions for ASCII characters."><code class="xref py py-mod docutils literal"><span class="pre">curses.ascii</span></code></a> module supplies ASCII class membership functions that
take either integer or 1-character string arguments; these may be useful in
writing more readable tests for such loops.  It also supplies
conversion functions  that take either integer or 1-character-string arguments
and return the same type.  For example, <a class="reference internal" href="../library/curses.ascii.html#curses.ascii.ctrl" title="curses.ascii.ctrl"><code class="xref py py-func docutils literal"><span class="pre">curses.ascii.ctrl()</span></code></a> returns the
control character corresponding to its argument.</p>
<p>There’s also a method to retrieve an entire string,
<a class="reference internal" href="../library/curses.html#curses.window.getstr" title="curses.window.getstr"><code class="xref py py-meth docutils literal"><span class="pre">getstr()</span></code></a>.  It isn’t used very often, because its
functionality is quite limited; the only editing keys available are
the backspace key and the Enter key, which terminates the string.  It
can optionally be limited to a fixed number of characters.</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">curses</span><span class="o">.</span><span class="n">echo</span><span class="p">()</span>            <span class="c1"># Enable echoing of characters</span>

<span class="c1"># Get a 15-character string, with the cursor on the top line</span>
<span class="n">s</span> <span class="o">=</span> <span class="n">stdscr</span><span class="o">.</span><span class="n">getstr</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span><span class="mi">0</span><span class="p">,</span> <span class="mi">15</span><span class="p">)</span>
</pre></div>
</div>
<p>The <a class="reference internal" href="../library/curses.html#module-curses.textpad" title="curses.textpad: Emacs-like input editing in a curses window."><code class="xref py py-mod docutils literal"><span class="pre">curses.textpad</span></code></a> module supplies a text box that supports an
Emacs-like set of keybindings.  Various methods of the
<a class="reference internal" href="../library/curses.html#curses.textpad.Textbox" title="curses.textpad.Textbox"><code class="xref py py-class docutils literal"><span class="pre">Textbox</span></code></a> class support editing with input
validation and gathering the edit results either with or without
trailing spaces.  Here’s an example:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">curses</span>
<span class="kn">from</span> <span class="nn">curses.textpad</span> <span class="k">import</span> <span class="n">Textbox</span><span class="p">,</span> <span class="n">rectangle</span>

<span class="k">def</span> <span class="nf">main</span><span class="p">(</span><span class="n">stdscr</span><span class="p">):</span>
    <span class="n">stdscr</span><span class="o">.</span><span class="n">addstr</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="s2">&quot;Enter IM message: (hit Ctrl-G to send)&quot;</span><span class="p">)</span>

    <span class="n">editwin</span> <span class="o">=</span> <span class="n">curses</span><span class="o">.</span><span class="n">newwin</span><span class="p">(</span><span class="mi">5</span><span class="p">,</span><span class="mi">30</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span><span class="mi">1</span><span class="p">)</span>
    <span class="n">rectangle</span><span class="p">(</span><span class="n">stdscr</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span><span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="o">+</span><span class="mi">5</span><span class="o">+</span><span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="o">+</span><span class="mi">30</span><span class="o">+</span><span class="mi">1</span><span class="p">)</span>
    <span class="n">stdscr</span><span class="o">.</span><span class="n">refresh</span><span class="p">()</span>

    <span class="n">box</span> <span class="o">=</span> <span class="n">Textbox</span><span class="p">(</span><span class="n">editwin</span><span class="p">)</span>

    <span class="c1"># Let the user edit until Ctrl-G is struck.</span>
    <span class="n">box</span><span class="o">.</span><span class="n">edit</span><span class="p">()</span>

    <span class="c1"># Get resulting contents</span>
    <span class="n">message</span> <span class="o">=</span> <span class="n">box</span><span class="o">.</span><span class="n">gather</span><span class="p">()</span>
</pre></div>
</div>
<p>See the library documentation on <a class="reference internal" href="../library/curses.html#module-curses.textpad" title="curses.textpad: Emacs-like input editing in a curses window."><code class="xref py py-mod docutils literal"><span class="pre">curses.textpad</span></code></a> for more details.</p>
</div>
<div class="section" id="for-more-information">
<h2>For More Information<a class="headerlink" href="#for-more-information" title="Permalink to this headline">¶</a></h2>
<p>This HOWTO doesn’t cover some advanced topics, such as reading the
contents of the screen or capturing mouse events from an xterm
instance, but the Python library page for the <a class="reference internal" href="../library/curses.html#module-curses" title="curses: An interface to the curses library, providing portable terminal handling. (Unix)"><code class="xref py py-mod docutils literal"><span class="pre">curses</span></code></a> module is now
reasonably complete.  You should browse it next.</p>
<p>If you’re in doubt about the detailed behavior of the curses
functions, consult the manual pages for your curses implementation,
whether it’s ncurses or a proprietary Unix vendor’s.  The manual pages
will document any quirks, and provide complete lists of all the
functions, attributes, and <code class="xref py py-const docutils literal"><span class="pre">ACS_*</span></code> characters available to
you.</p>
<p>Because the curses API is so large, some functions aren’t supported in
the Python interface.  Often this isn’t because they’re difficult to
implement, but because no one has needed them yet.  Also, Python
doesn’t yet support the menu library associated with ncurses.
Patches adding support for these would be welcome; see
<a class="reference external" href="https://devguide.python.org/">the Python Developer’s Guide</a> to
learn more about submitting patches to Python.</p>
<ul class="simple">
<li><a class="reference external" href="http://invisible-island.net/ncurses/ncurses-intro.html">Writing Programs with NCURSES</a>:
a lengthy tutorial for C programmers.</li>
<li><a class="reference external" href="http://linux.die.net/man/3/ncurses">The ncurses man page</a></li>
<li><a class="reference external" href="http://invisible-island.net/ncurses/ncurses.faq.html">The ncurses FAQ</a></li>
<li><a class="reference external" href="https://www.youtube.com/watch?v=eN1eZtjLEnU">“Use curses… don’t swear”</a>:
video of a PyCon 2013 talk on controlling terminals using curses or Urwid.</li>
<li><a class="reference external" href="http://www.pyvideo.org/video/1568/console-applications-with-urwid">“Console Applications with Urwid”</a>:
video of a PyCon CA 2012 talk demonstrating some applications written using
Urwid.</li>
</ul>
</div>
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<li><a class="reference internal" href="#">Curses Programming with Python</a><ul>
<li><a class="reference internal" href="#what-is-curses">What is curses?</a><ul>
<li><a class="reference internal" href="#the-python-curses-module">The Python curses module</a></li>
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<li><a class="reference internal" href="#starting-and-ending-a-curses-application">Starting and ending a curses application</a></li>
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<li><a class="reference internal" href="#displaying-text">Displaying Text</a><ul>
<li><a class="reference internal" href="#attributes-and-color">Attributes and Color</a></li>
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